In most videotex systems, text is received over the telephone line and displayed on a raster scan display (typically a television receiver). Character fonts used for raster scan display devices are usually represented by a matrix of binary bits and displayed as a matrix of black and white dots. The black/white dot matrix representing the video characters is typically derived from the corresponding character representation used in hard copy devices.
A problem arises, however, when the black/white dot matrix is used for video characters. The problem arises because conventional cathode ray tube (CRT) systems use an interlaced-field display format. In such an interlaced-field format, when a white character is displayed on a black background, or vice-versa, an annoying flicker results in the displayed symbol. Flicker results when two adjacent scan lines at the black/white vertical transition (horizontal edge) of the character, each in a different field of the display, are at a much different brightness level. Thus, for example, flicker results in a television receiver when a white scan line is followed approximately 1/60 of a second (field time) later by a black scan line. The combination of the large contrast and the time delay creates the annoying flicker. Flicker is undesirable since it causes the viewer to experience eye fatigue after prolonged viewing of the display.
It is known in the art to reduce the flicker caused in interlace-field displays by using a superposition technique or by using a scan line repeating technique. Superposition of the field removes the flicker, but results in a loss of vertical resolution. Repeating the data in the adjacent lines also reduces the visibility of line structures.
Low pass filtering can also be utilized to reduce flicker. For example, U.S. Pat. No. 3,953,668, issued on Apr. 27, 1967 to C. N. Judice, entitled "Method and Arrangement for Eliminating Flicker in Interlaced Ordered Dither Images" describes an area weighting method which averages the intensity of a group of cells on adjacent scan lines.
Additionally, U.S. Pat. No. 3,192,315, issued on June 29, 1965, to W. R. Remley and entitled "Two Dimensional Bandwidth Reduction Apparatus for Raster Scanning Systems" describes apparatus for smoothing the contrast of a raster symbol in both the direction of the scan and in the direction perpendicular to the direction of the scan. Both of the above patents, however, result in a sacrifice in sharpness of the entire picture to accomplish a reduction in flicker. Additionally, the Remley patent produces lines of non-uniform intensity which distorts the size and shape of the characters resulting in a reduction in the legibility of the characters.
Grey level character fonts have also been used to reduce flicker in CRT displays. For example, in Proceedings of the Conference SIGGRAPH'80, an article entitled "The Display of Characters Using Grey Level Sample Arrays", by J. E. Warnock, dated July, 1980, pp. 302-307 and SID Digest, an article entitled "Soft Fonts", by N. Negroponte, dated 1980, pp. 184-185. However, these type of arrangements require the storage, at the receiver, of grey level signals representing each received black/white character. Considerable memory is required to store the multi-level grey character signals.
Thus, there is a continuing need in the art to reduce the flicker in characters displayed on a interlace-field display devices (CRT) without sacrificing the sharpness of the entire picture being displayed or the need for large character memories.